This invention relates to a field assembly for an electromagnet and, more particularly, to a field assembly for use in an electromagnetic coupling such as an electromagnetic clutch or brake. A field assembly for an electromagnetic coupling is disclosed in Olsen U.S. Pat. No. 4,210,890.
In general, a field assembly for an electromagnetic coupling comprises a bobbin molded of plastic and defined by a central tubular spool and by a pair of annular flanges which project radially outwardly from the ends of the spool. A coil is supported by the bobbin and is formed by multiple turns of comparatively small diameter magnet wire wound on the spool between the flanges. Typically, a terminal block is molded integrally with and projects axially from one of the flanges of the bobbin. The two ends of the coil of magnet wire are either connected electrically to metal terminals on the terminal block or are spliced to larger diameter and heavily insulated lead wires which extend from the terminal block.
Prior field assemblies also comprise a shell stamped from steel or other material having a low magnetic reluctance. The shell supports the bobbin and defines magnetic poles. When the coil is excited by a voltage source, magnetic flux threads through the shell and creates a magnetic field which, in most electromagnetic clutches and brakes, serves to attract a rotating armature into frictional engagement with a coacting member so as to engage the clutch or brake.
Electromagnetic clutches are widely used in conjunction with air conditioners for vehicles and serve to selectively establish a drive between the air conditioner compressor and the vehicle engine. When used in such applications, the clutch must be capable of transmitting high torque and yet must be radially and axially compact and light in weight. Also, the clutch must be reliable and capable of experiencing a long service life.
Cost is a major factor in the design of an electromagnetic clutch for an air conditioner compressor. Different compressors usually require different clutches and, even if the same basic clutch is used with the same compressor, different electrical terminal connections are often required when the compressor is used with different makes and models of vehicles. It is very costly to provide basically different tooling and basically different manufacturing and assembly equipment for several different versions of clutches. If a given version of a clutch is required in only relatively low volumes, the tooling cost can make the clutch prohibitively expensive.